For measurements in the field of optical metrology, wavelength-stabilized gas lasers (HeNe lasers) are frequently used as the light source. Said gas lasers essentially have a high wavelength stability (depending on the stabilization method) and a large coherence length of a few hundred meters. As a result, said beam sources are particularly suitable for use as frequency and wavelength standard and enable measurement ranges which are large for interferometric measurement systems. Typical uses include, for example, linear interferometers, wavelength standard, vibrometer, and the use as interferometer light source in a laser tracker.
However, one disadvantage of the use of gas laser sources (HeNe laser light sources) is, as concerns a generally desired miniaturization, in particular of laser trackers, the dimensioning thereof defining the light output. The output of the light source here significantly depends on the length of the laser tube, i.e. the longer the tube, the greater the obtainable emission performance. In addition, such a laser source usually exhibits a relatively large power dissipation. Another disadvantage is the high-voltage supply required for operation. For example, for the laser to ignite, a voltage of approximately 7000 V and, during operation, a voltage of approximately 1500 V must be provided, as a result of which, during use of such light sources, special components (for example high-voltage power supply and shield) must be used and safety measures must be taken. Even the sensitivity with respect to magnetic fields (for example produced by internal motors or external welding transformers) and the limited lifetime of the tubes (typically about 15,000 operating hours) make the use of HeNe lasers a disadvantage—for example because the light sources must frequently be replaced in the systems at great expense.
Alternative light sources are, in this context, for example laser diodes. They are generally compact, cost-effective and have a low power consumption. Conventional Fabry-Perot laser diodes, however, are not suitable as interferometer light sources since they have a relatively small coherence length and do not emit in single-mode fashion (longitudinally) (i.e. emit with a plurality of wavelengths).
Beam sources which can be used are, however, for example                distributed feedback laser (DFB) (with a periodically structured active medium, for example a grating),        distributed Bragg reflector laser (DBR) (with an optical grating outside the active medium but arranged on a common chip),        fiber Bragg grating laser (FBG) (substantially in accordance with a DFB laser, but with a grating in an external fiber),        external cavity diode laser (ECDL) (stabilization of the laser diode using an external highly stable cavity, for example with a holographic grating),        diode pumped solid state lasers (DPSS),        discrete mode lasers (DMD) and/or        microchip lasers.        
The beam sources are here configured such that the emitted laser beam is single-mode with respect to the wavelength with a coherence length in the order of magnitude of multiple 10 m (or a linewidth of <1 MHz).
For the use of such laser diodes as an interferometer light source or as a wavelength standard, additionally some stable-holding of a specific wavelength is required. As is known, this can be effected for example spectroscopically using an absorption line of an absorption medium (for example using a gas cell). The disadvantage of using an absorption cell for stabilizing is, in turn, the required space associated therewith.
Alternatively, any wavelength could in principle simply be set and, during production, be identified by means of an external wavelength measuring appliance. If the diode parameters set therefor, such as for example temperature and current, are stored and are restored when it is next switched on, the original wavelength should be obtained again. However, the implementation of this stabilization capability is difficult for example on account of ageing effects of the diodes and a wavelength change which is caused thereby and continues to have uncertainties with respect to the emitted wavelength.
The requirements for such a measurement appliance are transferable analogously to measurement apparatuses which have an interferometer unit for determining distance changes. Here, measurement apparatuses which are configured for continuous tracking of a target point and coordinative determination of the position of this point can be combined generally under the term laser tracker. A target point can in this case be represented by a retroreflective unit (for example cube prisms), at which an optical measurement beam of the measurement apparatus, in particular a laser beam, is aimed. The laser beam is reflected back to the measurement apparatus in a parallel manner, wherein the reflected beam is captured with a capturing unit of the apparatus. Here, an emission or reception direction of the beam is ascertained, for example using sensors for angular measurement which are assigned to a deflection mirror or a targeting unit of the system. In addition, by capturing the beam, a distance of the measurement appliance to the target point is ascertained, for example using time-of-flight or phase difference measurement.
Laser trackers according to the prior art can additionally be configured with an optical image capture unit having a two-dimensional light-sensitive array, for example a CCD or CID camera (CCD=charge coupled device; CID=charge injection device) or a camera based on a CMOS array, or having a pixel array sensor and an image processing unit. The laser tracker and the camera are here in particular mounted on top of one another such that their positions relative to one another are unchangeable. By way of example, the camera is arranged so as to be rotatable together with the laser tracker about the substantially perpendicular axis of the latter, but so as to be able to pivot up and down independently of the laser tracker and hence so as to be separate from the optics of the laser beam, in particular. In particular, the camera can have fish-eye optics and thus pivoting of the camera owing to a very large image capturing region of the camera can be avoided or at least be necessary in a limited fashion. In addition, the camera can be configured—for example in dependence on the respective use—to be pivotable about only one axis. In alternative embodiments, the camera may be installed in an integrated design together with the laser optics in a common housing.
By capturing and evaluating an image—using image capture and image processing units—of what is known as an auxiliary measuring instrument with markings whose relative positions with respect one another are known, it is possible to deduce an alignment of the instrument and of an object (for example a probe) arranged on the auxiliary measuring instrument in space. Together with the determined spatial position of the target point, it is furthermore possible to precisely determine the position and alignment of the object in space absolutely and/or relative to the laser tracker.
Such auxiliary measuring instruments can be embodied by what are known as contact sensing tools that are positioned with their contact point on a point of the target object. The contact sensing tool has markings, e.g. light dots, and a reflector, which represents a target point on the contact sensing tool and can be targeted using the laser beam from the tracker, with the positions of the markings and of the reflector relative to the contact point of the contact sensing tool being precisely known. In a manner known to a person skilled in the art, the auxiliary measuring instrument may also be a scanner, for example handheld, equipped for distance measurement for contactless surface surveying operations, with the direction and position of the scanner measurement beam used for the distance measurement relative to the light dots and reflectors that are arranged on the scanner being exactly known. Such a scanner is described in EP 0 553 266, for example.
Furthermore, in modern tracker systems—increasingly as standard—a sensor (PSD) is used to ascertain a deviation in the received measurement beam from a zero position. In this connection, a PSD is intended to be understood to mean an area sensor that operates locally in the analog domain and that can be used to determine a focus for a light distribution on the sensor area. In this case, the output signal from the sensor is produced by means of one or more photosensitive areas and is dependent on the respective position of the light focus. Downstream or integrated electronics can be used to evaluate the output signal and to ascertain the focus. In this case, the position of the focus of the impinging light dot can be ascertained very quickly (microsecond range) and with nanometer resolution.
This PSD can be used to determine a deviation in the impingement point of the captured beam from a servo control zero point, and the deviation can be taken as a basis for readjusting the laser beam to the target. For this purpose and in order to achieve a high level of precision, the field of view of this PSD is chosen to be comparatively small, i.e. to correspond to the beam diameter of the measurement laser beam. Capturing using the PSD takes place coaxially with respect to the measurement axis, as a result of which the capturing direction of the PSD corresponds to the measurement direction.
For distance measurement, laser trackers in the prior art have at least one distance measuring device, said distance measuring device possibly being in the form of an interferometer, for example. Since such distance measurement units can measure only relative distance changes, what are known as absolute distance meters are installed in today's laser trackers in addition to interferometers. By way of example, such a combination of measuring means for distance determination is known by means of the product AT901 from Leica Geosystems AG.
The interferometers used for the distance measurement in this connection can—on account of the large coherence length and the measurement range permitted thereby—have HeNe gas lasers or the abovementioned laser diodes as light sources, which laser diodes have stated advantages in terms of power consumption and space requirement. A combination of an absolute distance meter and an interferometer for determining distance with an HeNe laser is known from WO 2007/079600 A1, for example. Use of a laser diode as an interferometer laser light source is described, for example, in European patent application no. 11187614.0.
For a reliable distance measurement or a measurement of the distance change during use of the laser diode that is desired with respect to the above-mentioned advantages, here the wavelength of the measurement radiation used must be kept stable and be known precisely (in particular to within a few picometers). Reproducing such a determined and thus known wavelength using defined driving of the diode can in this context not take place with absolute reliability.